Pressurized water nuclear reactors comprise a vessel which is filled, when the reactor is in operation, with a coolant fluid which consists of water under pressure. The reactor core, consisting of fuel assemblies, is completely immersed in the coolant water.
In the use of nuclear reactors such as pressurized water nuclear reactors, measurements must be made in the core while the reactor is operating. It is necessary, in particular, to carry out measurements of neutron flux at various places along the height of the core, in order to determine the neutron flux or power distribution along the axial direction of the core, which generally corresponds to the vertical direction.
In their central part, the fuel assemblies forming the core comprise an instrumentation nozzle into which can be introduced, throughout the height of the core, a leakproof thimble, inside which a neutron flux measurement probe is moved while the reactor is operating.
The thimbles associated with each of the fuel assemblies in which flux measurements are carried out must be capable of being withdrawn from these assemblies, for example when reloading of the core is carried out. These thimbles are therefore mounted so that they can move and slide inside guide tubes connecting the lower part of the vessel to a measurement room or instrumentation room in which the ends of the thimbles remote from the ends which are introduced into the core are accessible for the movement of the probes, for the collection of the measurement signals and for the movement of the thimbles inside the guide tubes. The movement of the thimbles in the guide tubes is performed merely by pushing or pulling, sufficient clearance being provided to limit the forces to be applied to the thimble, despite the curved shape of the guide tubes over most of their length.
In addition to the movable neutron flux measurement probes, use is also made of a set of flux-measuring devices such as collectrons, placed in a stationary position in the core and at various levels along its height.
Devices for measuring temperature, pressure or level, which are placed in the core at various determined locations, must also be used while the reactor is operating.
In order to simplify the instrumentation of the core and the procedures for using and maintaining this instrumentation, it has been proposed to place all the measuring devices relating to a core assembly in which measurements are carried out inside leakproof supporting and positioning conduit designed similarly to the conduit receiving the movable probe. A conduit of this kind comprises an outer tubular enclosure in which there is mounted, in a central position, coaxially with the enclosure, a sheath or thimble permitting the guidance of a movable probe for measuring neutron flux. Measuring devices such as collectrons are placed in the conduit, around the central sheath and in stationary positions distributed along the length of the conduit. The conduit for supporting and positioning the measuring devices has a diameter which is slightly smaller than the diameter of a guide tube permitting its movement and connecting the lower part of the vessel to the instrumentation room. The conduit may be placed in the core or withdrawn from the core from the instrumentation room, merely by its end being pushed or pulled. Inside the instrumentation room, the measurement conduit is mounted in a sliding and leakproof manner inside a nozzle which is rigidly integrally attached to the end of the corresponding guide tube.
It is obviously necessary to employ a central thimble and a neutron flux measuring probe which are miniaturized in relation to the prior art, the intrumentation nozzles of the assemblies being the same as those which were intended to receive thimbles alone. Similarly, the passages reserved for the conduits for supporting and positioning measuring devices in the internal structures of the reactor vessel, between the bottom of the vessel and the base of the core, as well as the guide tubes, are the same as those which were employed for guiding the thimbles alone.
These measurement conduits or leads must have sufficient flexibility to be capable of being moved without excessive force by pushing and by pulling inside curved guide tubes connecting the instrumentation room to the bottom of the vessel. The outer enclosure of these measurement conduits or leads must, furthermore, withstand major forces in a fluid at a high temperature and at a high pressure and under irradiation. In particular, this leakproof outer enclosure must withstand the pressure of the primary coolant water which is exerted on its wall while the reactor is in operation.
To avoid subjecting the wall of the measurement conduit to the pressure of the reactor's primary coolant fluid it has been proposed, in applicant's French Patent Application No. 86-16,505, to produce the measurement conduit in such form that it is in contact with the primary fluid on both its outer surface and its inner surface. The measurement conduit is made in the form of an tubular unit comprising lengths of flexible tubing which are connected together by spacers in which peripheral measuring probes are placed. The central thimble placed coaxially relative to the conduit is made in a leakproof form and such as to withstand the pressure of the primary fluid with which it is in contact via its outer wall. The peripheral measuring probes are immersed directly in the primary fluid inside the measurement conduit. The position of the peripheral probes along the length of the conduit is perfectly determined by the position of the spacers connected by variable lengths of flexible metal tubing.
A device of this kind offers the advantage of being capable of being produced in a form which is both flexible and robust and of facilitating placement of the instrumentation in the reactor core.
However, in the case where a peripheral measuring probe has become defective in operation, the entire measurement conduit must be withdrawn from the corresponding guide tube and must be repaired in a workshop, in order to replace it.
Furthermore, the position of the probes cannot be modified, the latter being determined once and for all when the measurement conduit is designed.
Lastly, the guide tubes towards the various assemblies in which measurements are carried out are of different lengths and require customizing of the design of the measurement conduits. This necessity creates problems which are difficult to solve in the context of the manufacture, storage and replacement of the measurement conduits.